Electric furnaces used for the melting of scrap iron or other raw material and the reduction of the bath of metal obtained, with optional addition of alloy elements until a metal of determined composition is obtained, have long been known in the art.
An electric furnace is generally constituted by a vessel delimited by a lateral wall and a bottom covered with a sole of refractory material and closed with a removable dome-shaped lid.
At least one electrode, called consumable, generally constituted by a bar of graphite, penetrates inside the vessel. Each electrode is supported by an arm extending above the dome and is mounted for vertical movement, so as to pass through the dome and descend into the raw material loaded into the furnace. This raw material, which is generally constituted by scrap iron, is in contact with at least one fixed electrode passing through the bottom and the refractory sole.
In the case of a single-phase alternating current furnace or of a direct current furnace, the consumable electrode and the sole electrode are connected to the two poles of a current source.
In the case of a two- or three-phase alternating current furnace, the consumable electrodes are connected to the poles of the current source and the charge is in contact with the sole electrode which constitutes the neutral of the system.
There are thus formed between the charge and each consumable electrode one or more electric arcs, which produce the melting of the scrap iron and the formation of a bath of metal in the bottom of the vessel.
Until recently, furnaces supplied with alternating current in particular were used, but it was found that the supply of direct current to the electrodes brought a number of advantages such as reduced noise and increased energy efficiency because it is possible to use higher voltages than those permitted with alternating current.
Provided that the charge is in scrap-iron form, the electrodes penetrate the latter by digging holes in it which determine a sort of insulation of the arcs from one another and tend to stabilize them. On the other hand, when the charge is entirely melted, the arcs which are subject to the magnetic effects created by the current passing through the electrodes, through the conductors which connect the electrodes to the current source and through other parts of the installation, may form in unpredictable directions and are therefore very unstable.
The zone in which the arcs form, which is at the highest temperature, can therefore not be kept at the center of the furnace, the walls and bottom of which may be subjected to excessive temperatures and substantial wear of the refractory lining.
Applicants have already disclosed, in U.S. Pat. No. 4,821,284, a process for controlling the direction of the electric arcs in a direct current furnace, even at high power levels.
To this end, instead of separating the conductors as far as possible from the furnace in order to avoid the influence of the magnetic fields produced by the passing of the current, the conductors connected to the fixed electrodes are, on the contrary, made to pass along the lower face of the bottom and as close as possible to the latter determining the profile and orientation of the path followed by the conductors so that the passing of the current produces magnetic fields, the mutual effects of which, with respect to deviation of the arcs, and taking into account all of the magnetic influences exerted in operation by the other conductors and the various parts of the installation, are such, that the arcs tend to converge towards a predetermined zone of the bath of metal.
The intensities which can be made to pass in the conductors and the electrodes, in particular the sole electrodes, are limited, and several consumable electrodes and several sole electrodes must therefore be used. In U.S. Pat. No. 4,821,284, several arrangements were envisaged, using in particular three consumable electrodes and three fixed electrodes.
The arrangement and orientation of the conductors of the fixed electrodes are determined by a prior calculation taking into account the general arrangement of the installation and, in particular, of the electrodes. To this end, a mathematical model is made first permitting calculation of the effects on the arcs of all of the elements through which an electric current flows, taking into account the intensities, the magnetic characteristics of the various parts of the installation, of the changes in the charge and of the temperatures during the various phases of the melting operation, and distinguishing in the calculations between the fixed elements the characteristics and positioning of which are fixed in advance and the variable elements on which it is possible to act. The routing of the conductors can thus be determined by calculating in advance the foreseeable effects on the arcs.
Among the fixed elements must be cited in particular the general constitution of the furnace, such as the shape of the vessel and of its supports, the accessory components such as the taphole and its means of closure, the support arms of the consumable electrodes along which the corresponding conductors generally pass and the position of the current source which, generally, is placed as close as possible to the vessel so as to limit the length of the conductors.
In U.S. Pat. No. 4,821,284, the main concern was to act upon the routing of the conductors by calculating, by means of the mathematical model, the various magnetic effects resulting from it, so as to determine the optimum arrangement requiring conductors of minimum length for effective control of the directions of the arcs.
The invention patent thus permits the construction of high capacity direct current furnaces.
Up to a capacity of about 110 tons, it is possible to use a single consumable electrode, the intensity not exceeding about 100,000 Amperes. In that case, it is relatively simple, by means of the process set out above, to control the direction of the arc, and three electrodes are generally used, fixed in a symmetrical arrangement around the vertical projection of the consumable electrode.
However, above about 110 tons, intensities have to be used which require the number of electrodes to be increased with a consequent risk of disturbances resulting from the mutual effects of the fields produced by the flowing of the current in the large number of conductors. It then becomes very difficult to perfect the mathematical model in view of the large number of elements to be taken into account, and their interactions.
The studies conducted by applicants have, however, led to a particularly advantageous arrangement of the electrodes and conductors which permits construction of furnaces, having a very high capacity, capable of exceeding 110 tons while effectively controlling the direction of the electric arcs.